The long-term goal of this project is to understand how neurons of the cerebral cortex of mammals including ourselves communicate with each other by means of synaptic inhibition, in the belief that such basic understanding will lead to pharmacological tools that will therapeutically enhance such inhibitory processes in pathological conditions such as certain of the epilepsies. To approach this goal, we have examined in depth a non-classic form of inhibition that, unlike the better known type, is not mediated by GABAa receptors, and depends upon a potassium rather than chloride conductance. Most recently, we have discovered that the potassium conductance of this inhibitory effect (the late IPSP), as well as that activated by the GABAb agonist baclofen, is controlled by a GTP-binding protein (G-protein) of the sort that is a substrate for pertussis toxin. We therefore propose to use this agonist in a model system of cultured hippocampal neurons to begin to dissect the molecular control of the late IPSP conductance. We will use the single electrode voltage clamp to make necessary current measurements in neurons in the hippocampal slice, and whole cell patch clamp of cultured hippocampal neurons to make the quantitative intraneuronal injections and current recordings called for in Specific Aims II, III and V. Importantly, these studies are complemented by biochemical assays of molecular elements that underly this response. Specifically, we propose to: 1) further verify GABA, and to test somatostatin (the other presently remaining candidate as transmitter of the late IPSP) as transmitters of the late IPSP and as appropriate agonists with which to model the late IPSP conductance in culture; 2) to evaluate a model of the late IPSP conductance with respect to the critical characteristic of G- protein control, that is, to determine whether the potassium conductance that is elicited by GABAb agonists in cultured hippocampal neurons depends upon a pertussis toxin sensitive G- protein; 3) to specify, by examination of this model, the particular G-protein that controls this conductance, and the subunit of this protein that is responsible for its action; 4) to determine whether some of the molecular components of the late IPSP and the late IPSC itself are present in mammals other than the presently used rodent models; and 5) to further explicate the molecular organization of the control of the late IPSP.